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The underscore hack is valid. The validator is broken. The underscore hack is brilliant for one very important reason: it keeps your hacks simple and grouped with the property being replaced. Therefore, it’s easier to go back in the future and remove or change them if needed.

Child selectors are great, but they’re backwards from what we actually need: styles that only apply to IE for Windows, because it’s the one that’s broken. Creating new selectors somewhere after the original is hard to maintain because the hacks are no longer grouped by what’s being overridden.